Cardionics, Inc. of Houston, Texas is currently providing a relatively inexpensive portable pulsed Doppler (PD) ultrasonic unit that noninvasively measures cardiac output (CO) in humans. Using thermodilution as the invasive standard for CO determination, this study will compare the relative reliabilitities and accuracies of the PD device (CardioFlo) and a commercially available continuous wave Doppler unit (CWD) (Ultracom). The specific objectives of this study are to determine: 1) whether this simple non invasive device can accurately and reliably measure CO; and 2) to investigate possible design improvements that can be incorporated into a subseqent model. We will address the relative accuracies of several methods of obtaining and processing Doppler spectral signals. Quadrature outputs will be recorded and subsequently analyzed through computer based fast Fourier transform (FFT) and analog offset zerocrosser (ZC) units. Paired comparisons will be: 1) PD vs CWD; 2) baseband ZC vs FFT; 3) base-band ZC vs offset ZC; 4) analog or digital integration of the entire cardiac cycle vs left ventricular ejection period only; 5) analog temporal mean of consecutive cycles vs operator selected cycles. Phase II of this study will incorporate the results of Phase I into an improved CardioFlo unit which should have widespread commercial applications in clinical and research settings where CO is needed but the cost and risks of traditional invasive methods are not justified.